Colorless and pigmented oxidatively drying paints and coatings based on oxidatively drying oils, alkyd resins, epoxy esters and other oxidatively drying refined oils are known. These oils and binders crosslink oxidatively under the influence of oxygen (preferably atmospheric oxygen) by means of the addition of driers, such as metal carboxylates of transition metals. If this crosslinking takes place before the product is actually used, they can form a solid binder film, a skin, on the surface when stored in open or closed containers. This is highly undesirable and should therefore be avoided since it makes the paint more difficult to work with, and commonly interferes with the uniform distribution of the driers. The accumulation of the driers in the paint skin that forms can lead to considerable delays in the drying of the paint when it is applied.
Skinning in the paint film after application is also disadvantageous. Excessively rapid drying of the surface of the paint prevents the lower film layers from drying evenly because they are shielded from oxygen, which is prevented from sufficiently penetrating into and dispersing within the paint film. This can lead among other things to flow problems in the paint film, adhesion problems, or insufficiently hard films.
It is known to add organic substances to the paint that inhibit the reaction of the drier metal with (atmospheric) oxygen by binding the oxygen or by complexing of the drier metal. Formulations of known compounds that are suitable for this purpose can be found e.g. in H. Kittel “Lehrbuch der Lacke und Beschichtungen”, Colomb Verlag 1976; J. Bieleman “Lackadditive” Wiley VCH 1998 or Römpp Lexikon “Lacke und Druckfarben”, Thieme Verlag 1998.
Furthermore, DE-A 3 418 988 describes the use of aliphatic α-hydroxy ketones as anti-skinning agents, and DE-A 1 519 103 discloses N,N-dialkylated hydroxylamines for this purpose. Because of their low volatility, however, these agents can lead to severe delays in drying and often also to reduced film hardness values, so that their possible applications are limited. They have not been able to gain acceptance as anti-skinning agents.
EP-A 0 903 380 describes the use of α-keto-functionalised carboxylic acids and dicarboxylic acids and of derivatives thereof, optionally in combination with diketo compounds, as anti-skinning agents. A disadvantageous property of such products lies in their limited stability and hence restricted storage life.
Finally, pyrazole or imidazole derivatives, optionally in combination with aliphatic diketones, in particular acetylacetone, are recommended for this purpose in WO 00/11090. However, the cited aromatic amines can cause severe discoloration in the coating composition and also in the applied paint film and likewise give rise to severe delays in drying.
Oximes (in particular butanone oxime) or suitable phenolic compounds are mostly used today as anti-skinning agents in industry. The phenolic anti-skinning agents display a significant delay in surface drying, however, such that alone they are only suitable for certain coating compositions. Oximes such as e.g. methyl ethyl ketoxime or butyraldoxime, on the other hand, display only slight delays in surface drying due to their volatility. The most significant disadvantage of the oximes, which are widely used today, lies in their toxicity. In long-term inhalation studies on rats and mice, for example, an increased occurrence of liver tumors was observed after exposure to butanone oxime, as a result of which the German MAK committee has classified the substance as a category III A 2 carcinogen (MAK list 1997) [MAK=maximum allowable concentration in the workplace]. As a consequence of this, users have to observe elaborate personal protection precautions when working with paints containing oximes as anti-skinning agents.
It is an object of the present invention to develop organic compounds which prevent skinning in paints and coating compositions over an extended period and display only a slight delay in surface drying. Furthermore, the film hardness values obtained in the resulting films or coatings should not be negatively influenced and no discoloration should be caused. The products should display no disadvantageous toxicological properties.
It is another object of the invention that the organic compounds can be incorporated into many different oxidatively drying paints and coatings and by virtue of their physical properties can be used straight-forwardly and for many purposes in corresponding coating compositions.
Surprisingly it was found that these requirements could be achieved by the use of mixtures containing the organic compounds described below. In particular, the above-mentioned disadvantages of the specified hydroxylamines as anti-skinning agents could also be avoided by combining such substances with the additional compounds described below, and hence products that better satisfy requirements as anti-skinning agents are obtained.
In air-drying paints the mixtures according to the invention are suitable for preventing undesirable skinning, for improving the complete drying of paint films after application and for reducing the tendency of the applied paint film to discolor.